Ionic liquids (IL's) are considered as a relatively new and more environmentally benign generation of solvents for conducting chemical reactions or other processes than traditional volatile organic compounds (VOC's). ILs have high polarity, usually negligible vapor pressure and good solvent power for many organic and inorganic materials thereby facilitating chemical processes. Thus, they are potential replacements for VOCs in many industrial processes. However, many ILs are highly hygroscopic meaning that they readily absorb water from the atmosphere. It is well recognized that the presence of water in an IL can have a negative impact on its solvent properties. Furthermore, many procedures for isolating desirable products from ILs include adding large amounts of water which produces a solution of the IL in water. Because ILs are typically expensive materials, it is critical to provide an efficient and economical method for separating water from IL which will be effective with a wide range of water/IL compositions.
Kalb in US Patent Application Publication US2012/0116096 describes the utility of ionic liquids in many fields and the need for technology for removing water from IL. Kalb discloses adding an orthoester to react with the water contained in the ionic liquid and then removing the water/orthoester reaction products. However, this method results in the contamination of the IL with the orthoester and the reaction products.
Dissolution and processing of cellulose using ionic liquids is disclosed in U.S. Pat. No. 6,824,599 B2 (Swatloski, R. P. et al.). The reference relates to dissolving cellulose in an ionic liquid without derivatization, and regenerating it in a range of structural forms without requiring the use of harmful or volatile organic solvents. It also relates to controlling cellulose solubility and the solution properties by selecting the ionic liquid constituents, with small cations and halide or pseudohalide anions favoring solution.
Sun et al. (Green Chemistry, volume 11, pages 646-655, 2009) describe a process for upgrading biomass by dissolving wood in ionic liquids and then separately precipitating the cellulose and lignin components by the adding acetone/water and then removing the acetone. This process can contribute to the use of renewable resources. However, it leaves a solution of the ionic liquid in water. The authors point out that this process will require an industrially attractive method for removal of the water from the ionic liquid broth so that the ionic liquid can be fully recycled to reduce materials cost and so that the aqueous waste can be discharged in an environmentally acceptable manner.
Reverse osmosis (RO) and pervaporation are membrane separation methods that have been employed for drying of aqueous IL solutions. Du in “Membrane drying of ionic liquids” (PhD thesis, University of Toledo, December 2012) teaches dehydration using a commercial polyamide membrane by RO. Du's data presented in Table 1, below, shows that significant quantities of the IL (1-ethyl-3-methylimidazolium acetate) are lost in the permeate waste stream. Waste of expensive IL and the relatively high pressures required to achieve significant flux make this process uneconomical. Pervaporation mode membrane separations were also evaluated with commercial polysulfone membranes. Water flux decreased to zero at an IL content in the feed of 82%. A polyamide membrane was more successful at lower water concentration but showed a 150-fold decrease in water transport as the IL content of the feed increased from 81 to 97% (Table 2)
TABLE 1ConcentratedConcentratedPermeatePermeateRO PressureIL weightIL flow rateIL weightIL flow rate(psi)percent (%)(ml/min)percent (%)(ml/min)3505.448.50.301.54005.578.40.331.64506.478.20.331.9
TABLE 2IL FeedConcentrationWater flux(wt. %)(kg/hr/m2)80.60.01585.60.00989.40.00492.70.00194.50.000397.50.0001
The literature reports the effects of exposing another common type of membrane to an ionic liquid on the membrane separation factor. For instance, Garcia, et al. reported in the Journal of Membrane Science 444 (2013) 9-15 the effects of exposing a polyvinyl alcohol (PVA) membrane to an ionic liquid at various temperatures and for different times on the water/butanol separation factor by pervaporation. Pertinent results of this paper are shown in Table 3, below. When the PVA membrane is exposed to ionic liquid at 40° C. from 1 to 5 days, the water/butanol separation factor is at least 196. However, when exposed to the ionic liquid at 60° C. for only one day, the separation factor drops to 13.9 and declines slightly with longer exposures. At 80° C. exposure the separation factor drops below 11 and continues to decline with longer exposures. This strongly suggests that the PVA membranes are unstable in the presence of ionic liquids.
TABLE 3Ionic liquidexposureIonic liquidWater/Butanoltemperatureexposure timeSeparation(° C.)(days)Factor401196.9403207.2405241.360113.96039.46039.860310.160510.480110.98035.88053.6
It is known for example from Nemser et. al., U.S. Pat. No. 8,506,815, that pervaporation can remove water and methanol from conventional organic solvents using a fluoropolymer selectively permeable membrane. This patent does not disclose the separation of water from ILs. FIG. 1 is a semi-logarithmic plot of the activity coefficient (AC) of water in selected solvent/water mixtures as a function of respective solvent concentration in the mixture. Curves A-F are data for the water/tetrahydrofuran, water/acetic acid, water/isopropyl alcohol, water/ethyl alcohol, water/acetone, and water/methyl alcohol mixtures, respectively. Curve W is that of a typical water/IL (namely, water/1-ethyl-3-methylimidazolium acetate) mixture All of this plotted data except curve B was calculated using the Wilson model (Perry's Chemical Engineers' Handbook, 1999, pp 13-20, 13-21). for activity coefficient prediction. The ionic liquid/water solution data was determined from empirical measurements. FIG. 1 shows that mixtures of water and ILs have very different water activity coefficients than mixtures of water and common organic solvents. These data indicate that drying of ILs is much more difficult than drying of organic solvents.
Ethanol/water and tetrahydrofuran/water solution separation experiments by pervaporation were simulated at conditions of 80° C. with a Teflon® AF 1600 selectively permeable membrane copolymer of 65 mole % perfluoro-2,2-dimethyl-1,3-dioxole (PDD) and 35 mole % tetrafluoroethylene (TFE). Empirical permeation testing was performed to determine that this polymer achieves water permeance of 750 gas permeation units (GPU) and that the water/ethanol and the water/tetrahydrofuran selectivities are 15 and 50, respectively. A GPU is defined as 1 standard cm3/cm2-sec-cm Hg×10−6. The simulations further assumed initial ethanol and tetrahydrofuran feed concentrations were 50 wt %. Results are summarized in Table 4 and show relatively moderate selectivities with significant loss of solvent. This level of solvent loss would be too high for the economical recovery of expensive ionic liquids.
Thus, there is a clear need for a membrane process for separating water from ionic liquids which shows high flux for water with minimal permeation of the costly ionic liquids, which operates effectively over a wide range of ionic liquid/water mixture concentrations and which is completely stable to the aggressive ionic liquid-containing fluid medium over a range of 50 to 100° C.
TABLE 4Water/FinalWater/SolventSolventSolventwaterSeparationRecoverySolvent/Water SystemSelectivitywt %Factor%Ethanol/Water15206.177Ethanol/Water15513.367Tetrahydrofuran/Water50208.784Tetrahydrofuran/Water50513.379